Recently, it is demanded new trials for environmental preservation as important mission worldwide. Such mission is also demanded in the field of a lubricating oil, and it is further demanded a lubricating oil capable of reducing environmental load more than ever. As a lubricant oil capable of reducing the environmental load, a biodegradable lubricant oil draws attention, as the lubricant oil is susceptible to decomposition in natural world to reduce its effects on ecosystem even in the case that the lubricant oil is leaked out.
Many of biodegradable lubricant oils are used as a countermeasure in the case of leakage into livers and oceans. Its use is mandatory in some regions and applications. For example, in European countries, the use of the biodegradable lubricant oil is mandated in 2-cycle engine oil in an outboard motor for use in lakes regions, hydraulic oil for a construction machinery used near a liver for taking drinking water, or the like. In the United States, the use of the biodegradable lubricant oil is mandated in a lubricant oil used in wetted parts of a ship or the like.
Various kinds of studies have been performed as to the biodegradable lubricant oil described above. For example, according to patent document 1, it is disclosed a 2-cycle engine oil composed of polybutene, a polyol ester, a paraffin-based hydrocarbon solvent and an ashless detergent. According to patent document 2, it is disclosed a hydraulic oil, composed of a complex ester of a polyvalent alcohol, a straight-chain saturated fatty acid and a straight-chain saturated polycarboxylic acid, an antioxidant and a load-bearing additive and excellent in biodegradability, oxidation stability, wear resistance and low-temperature fluidity. According to patent document 3, it is disclosed a stern tube bearing oil, composed of a water-soluble (poly)alkylene glycol, a water-soluble thickener and a water-soluble rust prevention agent and excellent in compatibility with sea water, lubricating property and biodegradability.
Further, a biodegradable lubricant oil is frequently used at locations near water such as livers and oceans as described above. The lubricant oil is thus susceptible to contamination by water, so that it is necessary to sufficiently consider for preventing metal corrosion. Particularly in the case of sea water, a metal is easily susceptible to corrosion. Further consideration is necessary for a lubricant oil, which may possible be contaminated by sea water, for use in a ship, wind turbine on ocean, ocean current generator or the like. Among these applications, very high rust prevention performance against sea water is demanded in a stern tube bearing oil in a lubricant oil for a ship.